1951: Chapter 18 – Flanking and Overhead Fire

ALL SAFETY ALLOWANCES GIVEN IN THIS CHAPTER ARE THOSE ACCEPTABLE IN WAR. FOR SAFETY ALLOWANCES DURING PEACETIME TRAINING, REFERENCE SHOULD BE MADE TO INFANTRY TRAINING, VOL III, PAMPHLETS Nos. 31 AND 33, AND TO PERIODICAL ACIs LAYING DOWN SAFETY LIMITS ON TRAINING.

INTRODUCTORY NOTES

General

1. The provision of supporting fire to our troops is the main tactical role of the machine gun. The safety of the troops to be supported must be the first consideration of the machine gun commander.

Flanking or overhead

2. Supporting fire can be provided from the flank of a line of advance or defended locality, or by overhead fire, that is, when the trajectory passes over the heads of our own troops. Where possible, flanking fire positions should be sought; not only because of the greater fire effect usually obtained from the beaten zone in enfilade, but also because fire from a flank can be put down with safety considerably closer to the troops being supported than can overhead fire, it is necessary to determine that such fire will be safe to our own troops. This increases the time required for the guns to be brought into action.

Position of own troops

3. In order that the safety of the troops may be ensured, it is essential that their position or movements should be observed by or known to the fire controller. In defence, such observation or knowledge should not present any serious difficulty. In attack, the possibility of observing the movements of our own troops will depend on various factors, eg, the nature of the ground (whether open, close, flat or hilly) and obstructions to the field of view, (bad visibility, smoke screen, etc.). Since such observation can hardly be assured, it is evident that considerable caution will have to be exercised.

4. Apart from the above considerations, the machine gun, by reason its stable mounting and the close grouping of its fire, is well suited to carry out flanking or overhead fire with safety to our own troops.

Rules of safety

5. Flanking and overhead fire are governed by definite rules, which are contained in the following Lessons. There rules take into account unarmoured troops in the open. If our own troops are dug in, commonsense will indicate to what extent these rules can be relaxed. For instance, it may be safe to fire just over the top of a trench 200 yards in front of the guns. But at longer ranges, the risk of dropping bullets at a steep angle of descent into our weapon pits must be considered. Tanks are immune from machine gun fire, and fire may be put down close ahead of, or even among, friendly tanks.

6. On occasions it may be unsafe to engage a target if the fire control rules are complied with. It may, however, be possible to fire on the target by modifying the fire control rules, by reducing either the number of taps or the number of elevations.

LESSON 102.- FLANKING FIRE
A – INSTRUCTOR’S NOTES

Aim

1. To teach the rules of flanking safety.

2. To teach how to give the maximum possible support with safety to our own troops when advancing in the attack.

Class and instructors

3. Lecture – One officer instructor and not more than 30 in the class; Practice – Squads under squad instructors.

6. Lecture – Draw the diagrams given in Fig 26 on the blackboard; Practice – The instructor should select targets and positions of own troops and prepare problems before the lesson begins.

Approach

7. Give the aim of the lesson (see para 1 and 2 above).

8. State that flanking fire may be of two types:-

(a) Engaging a target towards which our own troops are advancing until it is no longer safe to fire.

(b) Laying a belt of protective fire as close in front of an infantry locality as is consistent with safety. This is dealt with in Lesson 113.

Rules

9. Tell the class that flanking fire is covered by six main rules which are stated and explained below.

10. RULE ONE.- THE POSITION OF OWN TROOPS MUST BE KNOWN OR THEY MUST BE WORKING TO A TIMED PROGRAMME.

This entails the observation of our own troops during the whole time that the guns are firing or the application of a timed programme based on a rate of advance which must not be exceeded by the infantry units concerned.

11. RULE TWO.- BARRELS MUST NOT POINT NOT BULLETS FALL WITHIN THREE DEGREES OF OWN TROOPS. (See Fig 26 below).

The lateral allowance of three degrees cover:-

(a) Minor inaccuracies in aiming, tapping and the estimation of the strength of side winds.

(b) Movement of the tripod settling in during firing.

(c) Half the width of the beaten zone.

When engaging a target, the allowance of three degrees must be measured from the edge of the target nearest to own troops. The only exception to this rule is in indirect fire, when engaging a target of less width that the gun frontage, the allowance is measured from the point on which the line of the gun nearest to own troops will fall.

Fire must cease when own troops reach the three degree limit of safety.

12. RULE THREE.- GUNS MUST NOT BE TAPPED WITHIN THE THREE DEGREE LIMIT.

This rule implies that the angle through which guns are tapped outside the edge of the target must be added to the basic three degree allowance. The allowance when engaging targets with width by direct fire and targets of equal or less width than the gun frontage by direct fire is 3 deg 15 mins. When engaging targets of greater width than the gun frontage by indirect fire the allowance will 3 deg 15 mins, 3 deg 30 mins, 3 deg 45 mins or 4 deg depending on the number of taps required to cover the width of the target. The allowance when engaging point targets by direct fire will be 3 deg 30 mins.

13. RULE FOUR.- THE THREE-DEGREE LIMIT EXTENDS TO A POINT 400 YARDS BEYOND THE CENTRE OF THE HIGHEST BEATEN ZONE AT RANGES UP TO 2300 YARDS AND TO A POINT 500 YARDS BEYOND THE CENTRE OF THE HIGHEST BEATEN ZONE AT RANGES ABOVE 2300 YARDS.

Example

Target – 1800 yards (R/F)

Highest elevation – 1850 yards.

3 deg limit extends to 2250 yards.

14. RULE FIVE.- CAREFUL ALLOWANCE MUST BE MADE FOR WIND.

If, when laying guns to engage a target, they are tapped, switched or elevated towards our own troops, the amount that they are tapped, switched or elevated must be added to the safety allowances (see examples below).

Example 1

Guns are engaging a point target with own troops approaching from the left. A wind is blowing from 8 o’clock at 10mph.

To hit the target, the gun must be tapped to the left one tap (8 o’clock wind at 10 mph requires 13 minutes allowance). This will bring it closer to our own troops. The safety allowance must therefore be:-

Basic angle: 3 deg

Method of fire: 30 mins

Wind allowance: 14 mins

Total allowance: 3 deg 45 mins

Fire must cease when troops are 3 deg 45 mins from the target.

Example 2

In the previous example, if the wind has been blowing from the right no extra allowance for wind would have been required.

To hit the target, the gun would have to be tapped to the right and away from our own troops. There would be therefore no necessity for increasing the allowance of 3 deg 30 mins.

15. RULE SIX.- THE SAFETY ALLOWANCE MUST BE MEASURED BY ACCURATE MEANS.

The safety allowance must NOT be measured by hand angles.

Practice

16. Revise the rules of flanking fire by questions.

17. Set a simple problem of 3 deg safety.

18. Discuss the problem.

19. Set problems involving the method of fire.

20. Discuss problems.

21. Set problems involving wind.

22. Discuss problems.

23. Further problems as required.

Conclusion

24. Questions to and from the class.

25. Sum up main points.

LESSON 103.- OVERHEARD FIRE – DIRECT
A – INSTRUCTOR’S NOTES

Aim

1. To teach the rules of overhead safety.

2. To teach how to give with safety the maximum possible support by overhead fire to our own troops advancing in the attack.

Class and instructors

3. Lecture – One officer instructor and not more than 30 in the class; Practice – Squads under squad instructors.

6. Lecture – Draw the diagrams given below on the blackboard; Practice – The instructor should select targets and positions of own troops and problems.

B – CONDUCT OF LESSON

Approach

7. Give the aim of the lesson (see para 1 and 2 above).

Rules

8. State the rules for overhead fire:-

(a) RULE ONE.- THE POSITION OF OWN TROOPS MUST BE KNOWN OR THEY MUST BE WORKING TO A TIMED PROGRAMME.

(b) RULE TWO.- TRIPODS MUST BE IN GOOD CONDITION AND BARRELS CHECKED WITH THE .306 GAUGE PLUG. (See Lesson 16).

(c) RULE THREE.- THE RANGE TO OWN TROOPS MUST NOT EXCEED 3800 YARDS.

(d) RULE FOUR.- THE RANGE TO OWN TROOPS MUST BE FOUND BY RANGEFINDER OR MAP OF SCALE NOT LESS THAN 1/25,000.

(e) RULE FIVE.- THE LOWEST TANGENT ANGLE TO ENGAGE THE TARGET MUST BE EQUAL TO OR GREATER THAN THE SAFETY ANGLE TO OWN TROOPS PLUS OR MINUS THE GROUND ANGLE.

(f) RULE SIX.- CAREFUL ALLOWANCE MUST BE MADE FOR WIND.

(g) RULE SEVEN.- THE GROUND ANGLE MUST BE MEASURED BY BINOCULARS, DIAL SIGHT OR OTHER ACCURATE MEANS.

Theory

9. Explain:-

(a) The safety angle.- For overhead fire to be carried out with safety, bullets must pass over the heads of own troops at certain minimum heights dependent on the range to own troops. These minimum heights can be expressed as angles and are called safety angles. Safety angles for all distances of own troops from the guns can be found in the range tables.The safety angles allows for:-

(i) Permissible errors in rangetaking.

(ii) Errors in estimating the strength of the wind.

(iii) Errors in laying and sighting.

(iv) Movement of the tripod settling in during firing.

(v) Depth of the lowest shot below the centre of the cone of fire.

(vi) The height of a lorry.

(b) The ground angle.- The ground angle is the angle between the line of sight to own troops and the line of sight to the target (see Fig 30 below).

The greater the ground angle, the closer can fire be laid to own troops. This is clearly shown in Fig 31 below.

(c)Near and far limits of safety.- When troops are advancing through the gun position to the target, under overhead fire, there must be a point after they have passed the gun position at which they are safe and a point nearer the target at which they again become unsafe. These points are called the near and far limits of safety (see Fig 32 below).

(d) The equivalent range.- The safety angle, which is in effect the angle between the line of sight to own troops and the trajectory giving minimum safety to own troops, is a form of tangent angle. Like a tangent angle, it can be expressed as a range. This is called the equivalent range (see Fig 33 below).Equivalent ranges for all distances of own troops from the guns are given in the range tables.

If a gun was laid on our own troops with the equivalent range on the tangent sight, the bullets would pass over their heads with the minimum clearance required by the safety angle. Conversely, if a gun was laid on a target with the range to the target on the tangent sight, the slide on the tangent sight could be moved up to the equivalent range for the range to the target. The line of sight through the sights would then meet the ground at a point which own troops would be safe. This would be the far limit of safety (see Fig 34 below).

Procedure

10. Describe how to determine the far limit of safety:-

The fire controller can employ either of the following methods:-

(a) The tangent sight method:-

(i) Lay a gun on the target with the lowest range that may be employed.

(ii) Without disturbing the laying of the gun, set the slide on the tangent sight at the equivalent range for the lowest range to be employed.

(iii) Note the point where the new line of sight meets the ground. Select a clearly defined object at that point or as near as possible on the gun position side of it. That is the far limit of safety and mire must cease when own troops reach that limit.

(iv) Move the sight back and check that the laying of the gun has not been disturbed.

The tangent sight method is the simpler and should be employed when the section commander has access to one of his guns. This method cannot be used on flat ground, when the target and far limit are on the same line of sight.

(b) The comparison of angles method.-

(i) From the range tables, obtain the tangent angle for lowest range to be employed to engage the target.

(ii) By eye, select a point which appears to be about the far limit of safety. Obtain the range to that point and look up its safety angle in the range tables.

(iii) Measure the ground angle and subtract it from the safety angle. Compare the result with the lowest tangent angle to be used. If it is equal to or slightly less than the lowest tangent angle, then you have found the far limit of safety.

(iv) If it is greater than the lowest tangent angle, another point should be selected nearer the gun line and the process repeated until the far limit is found.The comparison of angles method can be used on all occasions on which overhead fire is to be employed.Example:-

The point selected as the probable far limit of safety is at a range of 1215 yards, and the ground angle to this point is 30 minutes.

This angle being four minutes less than the lowest tangent angle, own troops are safe at this point, which is therefore the far limit of safety.

11. To determine the near limit of safety the procedure used is the same as for the far limit.

Example:- The point selected as the probable near limit was 50 yds from the gun position, and the ground angle to that point was 1 deg 15 mins.

Range to target

1650 yds (est).

Lowest elevation

1550 yds.

Tangent angle for 1550 yds.

2 deg 7 mins

Safety angle for 50 yds.

3 deg 17 mins

Ground angle for the point at 50 yds.

1 deg 15 mins

Therefore the safety angle minus the ground angle equals 2 deg 2 mins and this being five minutes less than the lowest tangent angle, own troops are safe, and the near limit of safety is therefore 50 yds in front of the guns.

NOTE.- If the ground is flat, no ground angles will come into the problem, and the overhead problem is consequently determined by a straightforward comparison of tangent and safety angles in the ranges table, the position on the ground being found after the ranges are known.

Assisting troops who have passed the far limit

12. State that when troops are advancing, they may be held up by enemy fire when they have passed the far limit of safety. On occasions, it may assist them to lay overhead fire as close as possible to them by means of the equivalent range. The procedure for this is to lay the guns on own troops with the equivalent range for the range to own troops on the tangent sight.

Head and rear winds

13. When engaging a target by overhead fire, a rear wind would tend to blow the bullets over the target. Consequently, the elevation on the guns must be lowered to hit the target. This might bring the lowest tangent angle to be employed lower than the safety angle to own troops, and thus endanger them if the wind suddenly drooped. Arrangements would therefore have to be made to bring the far limit of safety nearer to the guns.

A head wind would have to be offset by raising the elevation on the guns. Our own troops would not be endangered. The far limit of safety must not, however, be moved further away from the guns, as our own troops might be endangered.

14. The procedure for catering for head and rear winds is therefore as follows:-

(a) Tangent sight method.- The act of depressing the gun, by means of the handwheel, to offset a rear wind automatically brings the far limit nearer to the guns. When catering for a head wind, it would also move the far limit further from the guns. This must not take place. Therefore,

(b) Comparison of angles method.- As the calculations to determine the far limit are carried out by the fire controller mathematically, and not manually at the gun, the allowance for a rear wind must be made by the fire controller himself in determining the far limit of safety. Therefore,

Rear wind –

Use the equivalent range for the lowest range to be employed after wind correction.

Head wind –

Use the equivalent range for the lowest range to be employed. Do not correct for wind until after the far limit has been selected.

Rear wind –

Compare the lowest tangent angle to be employed after wind correction with the safety angle plus or minus the ground angle.

Head wind –

Compare the lowest tangent angle to be employed with safety angle plus or minus the ground angle. Do not make any allowance for wind when determining the far limit of safety.

Practice

15. Revise the rules of overhead safety.

16. Explain and demonstrate the method of obtaining the far limit of safety by the tangent sight method.

17. Practise the class.

18. Revise the method of obtaining the far limit of safety by the comparison of angles method.

19. Practise the class in problems.

20. Revise the method of obtaining the near limit of safety.

21. Practice the class in problems.

22. Practice the class in all types of overhead fire problems, including wind.

Conclusion

23. Questions to and from the class.

24. Sum up main points.

LESSON 104.- OVERHEAD FIRE – INDIRECT
A – INSTRUCTOR’S NOTES

Aim

1. To teach the method of giving overhead fire from an indirect fire position.

Class and instructors

2. Lecture – One officer instructor and not more than 30 in the class; Practice – Squads under squad instructors.

Periods

3. One 45-minute period, lecture; one 45-minute period, practice. This lesson should be taught after Chapter 19.

5. Practice – The instructor should select targets and positions of own troops and prepare problems before the lesson begins.

B – CONDUCT OF LESSON

Approach

6. Give the aim of the lesson (see para 1 above).

Rules

7. State that the rules for overhead fire – direct apply equally to indirect fire.

Far limit

8. Tell the class that the far limit of safety is obtained by the comparison of angles method.

Near limit

9. Explain that the near limit of safety is obtained as follows:-

(a) If the gun line is close to the crest, the near limit of safety is a point immediately beyond the crest behind which the guns are positioned, at which troops cannot be seen from the gun position.

(b) If the guns are well back from a shallow crest, the near limit may be on the same side of the crest. In this case the near limit can be obtained by the comparison of angle method. As no ground angle can be measured direct, the angle of sight to the target.Examples:- If the angle of sight to the target is greater than the angle of sight to own troops, the difference between them must be subtracted from the safety angle as in normal direct fire.If the angle of sight to the target is a plus angle of sight, but less than the angle of sight to own troops, the difference between them must be added to the safety angle.If the angle of sight to the target is a minus angle, the difference between that angle and the angle of sight to own troops must again be added to safety angle.

Protective fire

10. The procedure for laying protective fire, from an indirect position, as close to own troops as possible is as follows:-

(a) Obtain the range and angle of sight to own troops.

(b) Find from the range tables the equivalent range to own troops.

(c) Lay the guns by indirect means for direction on own troops.

(d) For elevation, order the equivalent range and the angle of sight to own troops. Fire can thus be laid as close as possible to own troops, consistent with safety.

Winds

11. Head and rear winds will have the same effect as in direct fire. Therefore:-

(a) When engaging a target,

For head winds, no allowance is necessary.

For rear winds, allowance must be made.

(b) When laying down protective fire,

For head winds, allowance must be made.

For rear winds, no allowance is necessary.

Practice

12. Practise class in far limit problems.

13. Point out a near limit, with guns close to crest.

14. Practise the class in near limit problems when the guns are well back from the crest.

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